1. Field of the Invention
The present invention relates to a semiconductor package such as a ball grid array (BGA), in particular, to a printed circuit board employed in such a semiconductor package.
2. Description of the Prior Art
In general, a BGA employs conductive through holes or vias so as to electrically connect a semiconductor chip, mounted on the front surface of a substrate, and solder balls attached to the back surface of the substrate. An increased number of input/output pins arranged on the semiconductor chip inevitably induces increased through holes. The increase of the through holes may hinder realization of a high-density and fine printed wiring pattern on the substrate. In addition, production efficiency may be deteriorated and cost in production may be increased.
For example, Japanese Patent Laid-open No. 8-31868 discloses a semiconductor package, a BGA, comprising a semiconductor chip and solder balls electrically connected to each other without employment of through holes. The disclosed BGA is designed to employ a printed circuit board comprising a folded insulated film of synthetic resin. A printed wiring pattern is formed over the surface of the insulated film. Electric connection can thus be achieved between the semiconductor chip and the solder balls via the printed circuit pattern, not through holes, in the BGA. However, the BGA may suffer from an insufficient rigidity of the printed circuit board. Stress applied to the BGA may induce deformation of the semiconductor chip, which possibly disconnects the electric connection between the semiconductor chip and the printed circuit board.
Another example of a BGA is disclosed in Japanese Patent Laid-open No. 8-204103. This BGA tries to maintain the rigidity of a printed circuit board even when a semiconductor chip and solder balls are electrically connected to each other without employment of through holes. The BGA is designed to employ a multilayered printed circuit board comprising a metallic plate, an insulator layer and a printed pattern film. The metallic plate serves to exhibit a higher rigidity. However, folding of the high rigidity metallic plate may deteriorate production efficiency.
It is accordingly an object of the present invention to provide a printed circuit board capable of electrically connecting a semiconductor chip and an input/output terminal on opposite surfaces of a substrate without employment of a through hole, at a lower production cost and a higher production efficiency.
According to the present invention, there is provided a printed circuit board for a semiconductor package, comprising: a substrate having a front surface and a back surface; an insulator film extending over the front surface and bent around an outer periphery of the front surface so as to reach the back surface; and a wiring pattern printed on the insulator film so as to extend over the front and back surfaces.
With the above structure, the wiring pattern serves to electrically connect the opposite surfaces, namely, the front and back surfaces of the printed circuit board. Employment of such a wiring pattern allows establishment of signal paths between the front and back surfaces of the printed circuit board without a conductive through hole or via. If the printed circuit board is employed in a semiconductor package such as ball grid array (BGA), for example, the wiring pattern may serve to facilitate establishment of electric connection between a semiconductor chip, mounted on the front surface of the printed circuit board, and a plurality of conductive balls such as solder balls and gold balls, formed or attached to the back surface of the printed circuit board. It is not required to form conductive through holes or vias. Even if input/output pins of the semiconductor chip are increased, it is possible to avoid a deteriorated efficiency and an increased cost in production.
A rigid body is preferably employed as a substrate. Such a rigid body may include a conventional resin plate, a copper plate, an aluminum plate, a ceramic plate, and the like, for example. When enough rigidity can be found in the substrate, the semiconductor package is reliably prevented from deformation. The semiconductor package can be grasped relatively hard. In addition, a wire bonding can be achieved easily.
The printed circuit board preferably further comprises a lining of a metallic thin film backing the insulator film. In the case where a synthetic resin is used to provide the insulator film, the metallic thin film may contribute to reinforcement of rigidity of the insulator film. If the synthetic resin insulator film is formed on the surface of the metallic thin film, the metallic thin film along with the insulator film can be rolled up onto a roll. Handling of the relatively fragile insulator film can be facilitated. It is preferable to keep enough flexibility in the metallic thin film even after the insulator film or membrane is coated on the surface of the metallic thin film.
In particular, when a metallic plate of a higher heat conduction is employed as the substrate, it is preferable to define an opening in the insulator film and/or the metallic thin film so as to expose the front surface of the metallic plate. If the semiconductor chip is disposed within the opening so as to directly contact the metallic plate, heat radiation from the semiconductor chip can be promoted through the heat conduction of the metallic plate. A heat sink, a heat pipe, or any other type of heat radiation mechanism may be added or attached to the metallic plate.
In addition, the conductive metallic thin film may be utilized as a ground pattern or layer. If the wiring pattern is connected to the ground pattern, a structure similar to a micro strip line can easily be established, so that a signal line can be prevented from suffering from noise crossing over adjacent signal lines.
When the aforementioned printed circuit board is to be produced, a method according to the invention may comprise: forming an insulator membrane on a front surface of a film; forming a conductive wiring pattern on a surface of the insulator membrane; attaching a core on a back surface of the film; folding a part of the film around an outer periphery of the core; and bonding the part at a back of the core, for example.
Such a method serves to establish signal lines between the opposite surfaces, namely, the front and back surfaces of the core without folding or bending the core itself. It is accordingly possible to avoid a deteriorated production efficiency due to folding or bending of the relatively high rigidity core. The core may include a metallic plate such as a copper plate and an aluminum plate in addition to a substrate such as a synthetic resin plate and a ceramic plate. The core need not be a rigid body in this case.
The film may take the form of ribbon. The film in the form of ribbon may be rolled up onto a roll after the conductive wiring pattern has been formed. Employment of the roll serves to facilitate handling of the film at factories.